The apparatus and method of the present invention are applicable to the production of parts, for example hardware such as nuts, bolts and other fasteners, which have been immersed in a bath of molten coating metal for applying a uniform coating of such metal to the surfaces of such parts.
Historically parts have been coated for decorative or protective purposes with a variety of metals, such as zinc and zinc alloys, aluminum and aluminum alloys, etc. The oldest and most common such coating metal is zinc, which has been in use for over 200 years for coating of large and small objects by a process known as hot-dip galvanizing. Generally, the application of a zinc coating to large objects presents fewer problems compared to those presented by the zinc coating of small objects. Such objects, such as fasteners, may have intricate shapes with crevices, holes or threaded portions within which the zinc coating builds up. Any such coating buildup creates problems from both an appearance and functional viewpoint.
In the prior art hot-dip galvanizing process, metal parts are placed into a basket which is immersed in a bath of molten zinc and then removed from the bath and spun in order to remove the excess zinc adhering to the parts. The smaller the part and the more intricate its shape the more difficult it is to remove the excess zinc coating. Defects apparent with the hot-dip galvanizing process are caused by too rapid cooling of the molten zinc due to the cooling resulting from the high velocity of air within the spinning chamber. The solidification of the molten zinc takes place rapidly before the excess zinc drains from the parts by centrifugal force and leads to non-uniform coating thickness. The fluidity of molten zinc on the surface of the fasteners is relatively small. The most common defects appear as too thin a coating on one side, i.e. front side, of the fasteners and solidified drops on the other side, i.e. back side. The latter defect is sometimes described as failure to spin-off the "last drop." In addition, the high velocity of the air during the spinning process also circulates small zinc particles within the spinning chamber. Coating metal remaining on the parts in the molten stage is subjected to impingement by the small particles of zinc, thus creating nucleations of zinc. Attempts have been made to promote uniform coating thickness and to reduce nucleation problems by sprinkling small amounts of fluxing chemicals on to the molten zinc to promote fluidity and to reduce oxidation. However, the boiling of the fluxing chemicals cools the meniscus of the excess zinc created at the touching points of parts resulting in the coated parts sticking together in clumps.
Several of the many patents directed to an invention which include a spinning or centrifuging step or spinning apparatus as part of a metallic coating process are as follows:
U.S. Pat. No. 1,773,495 to H. B. Newhall et al. utilizes a heated, non-oxidizing, neutral or reducing substance in a gaseous state circulated through a rotating screen positioned in a closed chamber and holding hot-dipped zinc coated parts.
U.S. Pat. No. 3,429,295 to Z. M. Shapiro is directed to a vapor deposition process for applying a coating on parts rolled in the vapor of the coating material on a rotating receptacle in a vacuum-tight chamber in a vacuum or inert atmosphere.
U.S. Pat. No. 1,418,942 to A. Gierek et al. is directed to a method of coating parts with a molten aluminum alloy by rotating a container holding the parts, within the range of 10-750 r.p.m., during its withdrawal from a bath of such alloy and then increasing the speed of rotation above the surface after withdrawal within the range of 10-1500 r.p.m.
U.S. Pat. No. 4,196,231 to E. Hubers describes apparatus and a method which includes vacuum impregnation of parts in a basket lowered into a tank of molten coating metal and subsequent spinning of the basket in air above the coating metal.
U.S. Pat. No. 4,526,231 to P. J. Sippola is directed to apparatus, i.e. a cage, for holding parts which are immersed in a zinc bath and then raised out of the bath into a centrifuging chamber having an oxygen-free atmosphere, such as nitrogen and rotated at a speed of 200-500 r.p.m.
Despite the claims made in the above mentioned patents about the quality achieved for the metallic coating applied to parts for decorative or protective purposes, the fact remains that presently available apparatus and methods for applying a metallic coating to metal parts for decorative or protective purposes do not produce a uniform coating and/or a coating without surface defects. The apparatus and method of this invention produces a metallic coated metal product with a uniform coating thickness and an improved surface condition.